This application is based on Application No. 2002-41648 filed in Japan on Feb. 19, 2002, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a method and an apparatus for indicating a battery state which indicates that a battery of hybrid car is charged/discharged in such a mild condition that elongates the battery life.
2. Background of the Related Art
A hybrid car is driven by an engine and a motor. The motor is driven by a battery. The battery which drives the motor is charged/discharged under very severe conditions, because it is discharged with a large current at the time of acceleration of a heavy vehicle, and charged with a large current at the time of breaking. In particular, at the time of hard acceleration and hard braking, the battery is charged/discharged with an extremely large current. For this reason, a battery of a hybrid car is requested to have long life although it is used under such severe conditions. This is because the battery is configured by a number of secondary batteries connected in serial, and hence the production cost is very high. The life of a battery is influenced by a driving condition of the hybrid car. For example, in the case of a driver who often uses hard acceleration and hard braking, the battery suffers great damage so that its useful life is shortened.
The life can be elongated by restricting the current with which the battery is charged/discharged. However, restricting the current for charging/discharging will result in decrease in the effect specific to a hybrid car which travels with the assistance of a motor and a battery, so that the advantage specific to the hybrid car is lost. Substantially this equals to miniaturizing the motor and the battery, and the traveling condition of the hybrid car is significantly deteriorated. For example in the case where the current for discharging is restricted, the acceleration performance is deteriorated, while in the case where the current for charging is restricted, the charging efficiency is deteriorated so that effective regenerative braking is impossible. Further difficulty is that since the condition of accelerating and braking differs depending on the driver, acceleration is least effective when a driver who drives gently operates the hybrid car. On the other hand, a driver who wants to accelerate hardly by hitting the accelerator feels such poor acceleration unsatisfactory. For this reason, in order to realize a desirable running condition in the hybrid car, it is impossible to restrict the current for charging/discharging. Consequently, the life of the battery is significantly shortened depending on the driver. In particular, since a driver cannot know the relationship between the driving condition and the life of the battery, she/he unconsciously shortens the life of the battery.
Furthermore, a hybrid car calculates the remaining capacity of the battery and controls charging/discharging so that the remaining capacity is in the vicinity of approximately 50% of the full-charge capacity. This is because charging/discharging at this remaining capacity provides the longest life of the battery. However, if an error occurs in the calculation of the remaining capacity, the remaining capacity of the battery is actually no longer charged/discharged in this range, so that the life of the battery is shortened. The remaining capacity can be corrected by battery voltage. This is because the battery voltage is one of the parameters for remaining capacity, as such the smaller the remaining capacity the lower the battery voltage becomes. Moreover, the remaining capacity can be corrected more accurately by means of current and voltage. This is because the battery voltage varies according to the magnitude of the current. When the remaining capacity is corrected by means of current and voltage, xcex94V/xcex94I which is a change value of voltage and current is detected. However, if the current for charging/discharging rapidly increases or decreases, it is impossible to detect xcex94V/xcex94I with accuracy. For this reason, it is impossible to accurately correct the remaining capacity from the xcex94V/xcex94I. Accordingly, an error is likely to occur in calculation of the remaining capacity, which results in reduction of the battery life.
The present invention was developed so as to eliminate the adverse effect as described above. It is a primary object of the present invention to provide a method for indicating a battery state of a hybrid car which indicates that the battery is used in a gentle environment wherein deterioration of battery is small, or that the battery is in the protective state, thereby enabling a driver to drive while being aware of the battery life.
A method for indicating a battery state of a hybrid car according to the present invention detects xcex94V/xcex94I which is a change value of voltage and current from a first state and a second state of a battery 1 being charged/discharged. The method for indicating a battery state determines whether the charging/discharging is performed in a battery protective state based on xcex94V/xcex94I and indicates that the battery protective state is established when the charging/discharging is performed in a protective state.
The method for indicating a battery state can determine that the battery protective state is established when the change in current for charging/discharging is smaller than a preset range.
An, apparatus for indicating a battery state of a hybrid car according to the present invention comprises a determination circuit 5 which detects xcex94V/xcex94I which is a change value of voltage and current from a first state and a second state of a battery 1 being charged/discharged and determines whether the charging/discharging is performed in a battery protective state based on xcex94V/xcex94I, and a display 6 for indicating that the battery protective state is established when it is determined that the battery 1 is charged/discharged in a protective state.
The above-mentioned method for indicating a battery state can indicate that the charging/discharging is performed in the condition of being used in a gentle environment involving less deterioration of battery, or the battery protective state, and enables the driver to drive while being aware of the battery life. This is because the above-mentioned method for indicating a battery state detects xcex94V/xcex94I of the battery being charged/discharged and determines whether the battery protective state is established using the xcex94V/xcex94I as a parameter. When the hybrid car is driven gently without frequent hitting of the accelerator pedal and the brake pedal, the change in charging/discharging current of the battery is reduced. In the condition that the change in charging/discharging current is small, xcex94V/xcex94I of the battery can be detected with accuracy. Therefore, by detecting xcex94V/xcex94I, it is possible to determine whether or not the driver drives the hybrid car gently. In addition, since a substantial internal impedance of the battery can be detected from xcex94V/xcex94I, it is possible to calculate the remaining capacity with accuracy by correcting the battery voltage using this internal impedance. It is also possible to protect the battery so as to be charged/discharged at an optimum remaining capacity by Judging overcharge and over-discharge of the battery using the value of the detected xcex94V/xcex94I or detecting deep charging/discharging.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.